Hydrometallurgy is a process for separating valuable metallic species from other less valuable materials. The process involves the dissolution of the valuable metallic species into an aqueous solution, which is then separated from the insoluble residue. To enhance the rate of ion dissolution and to increase the loading of metal ions in the solution, it is common practice to use an acidic or basic solution. An example of a particularly useful basic solution is a mixture of sodium hydroxide in water. Other alkali materials can also be utilized, but the relatively low cost of sodium hydroxide usually makes it the most economical choice.
The aqueous solution loaded with dissolved metals is referred to as a “pregnant liquor.” Dissolved metals may be recovered from the pregnant liquor by one or more means, including: electrolysis, neutralization, and immiscible solvent extraction.
Hydrometallurgical methods for recovery of valuable metals have been practiced for decades. The following discussions and examples are based on recovery of zinc oxide from a mixed feedstock material. The basic-soluble zinc oxide is separated from non-basic soluble materials. The non-soluble materials include (but are not limited to) metals and metal oxides such as iron, iron oxide, nickel, cobalt, precious metals, and non-metal oxides such as silica.
There are several processes identified in the literature for recovery of zinc from zinc-containing feedstock mixtures. These processes typically involve three generic steps:
1. Contacting the zinc-containing feedstock with dilute base to selectively solubilize the zinc, usually at elevated temperatures
2. Separating the leach residue from the basic solution by filtration, centrifugation or other means
3. Recovering zinc from the basic solution (pregnant liquor) by electrowinning, neutralization, or other means.
The most difficult step in this process is usually the separation of the leach residue from the pregnant liquor. The fine particles suspended in the pregnant liquor are very difficult to completely remove. Relatively high pregnant liquor viscosity and surface tension make the removal of these fine particles by filtration or centrifugation exceedingly slow. However, if the particles are not essentially completely segregated from the pregnant liquor, then they will contaminate the zinc-rich product in the next step, rendering the entire purification process useless.
An article entitled “Recovery of Lead and Zinc from Electric Steelmaking Dust by the Cebedeau Process”, by J. Frenay et al. summarizes commercial and pilot scale attempts to separate zinc from basic-insoluble species. The high viscosity of highly concentrated basic solutions typically limits commercial operations to a maximum concentration of about 25-30 weight percent base.
The cost of hydrometallurgical processing is heavily dependent on the loading or concentration of the dissolved metal species in the pregnant liquor. As the loading is increased, the amount of liquor that must be processed to produce a given amount of product decreases, saving both capital and operating expense.
Higher concentrations of base permit higher loadings of base-soluble metals in solution. However, higher concentrations of base also produce a significantly more viscous solution. This higher viscosity hinders down-stream processing including the separation of the pregnant liquor from the leach residue.
A number of processes have been developed to recover zinc from various waste materials using hydrometallurgy, but few have been commercially successful. In large part, this is due to the high cost of recovering the dissolved metal species from the pregnant liquor. Typical metal recovery strategies include:                Electrolysis where a flowing electrical current reduces the metal ions to the metal and plates the metal atoms onto an electrode.        Neutralization of the liquor to a near-neutral pH to precipitate various metallic salts, hydroxides, or oxides.        Extraction of metallic ions or complexes with an immiscible solvent.        
All of these methods of metal recovery are relatively expensive.                Electrolysis requires large amounts of electrical current to reduce the metal from a higher valence state to metal. Furthermore, if a metal oxide is the desired end-product, then the base metal must be subjected to an oxidation process to create the oxide form.        Neutralization of the pregnant liquor requires large quantities of reagent. The neutralization process effectively destroys the liquor for further extraction, and creates a waste salt stream that must be disposed of        Extraction with an immiscible solvent (such as kerosene doped with an organic amine) generally requires a large excess of extraction solvent, and costly post-processing to recover the metal from the immiscible solvent.        
U.S. Pat. No. 4,005,061 to Lemaire discloses a method of removing zinc from spent battery zinc/air electrolyte using a miscible solvent. The single material referenced in the '061 patent is characterized as a “waste,” however, this chemical system is, in fact, a spent material containing potassium hydroxide and potassium zincate plus a few percent of potassium carbonate and trace impurities. The described system is directed to electrochemical storage cell batteries having a zinc negative electrode and is, therefore, different from and substantially less complex than the metallurgical waste and by-product materials that are the subject of the present application. The electrolyte is spent only because the metallic zinc powder has been oxidized by air to potassium zincate. It has not been mixed with other materials and only one, simple chemical reaction has occurred. Metallurgical wastes and by-products, spent catalysts, etc., on the other hand, are typically complex mixtures containing a number of different chemical elements in significant concentrations, and they often contain a number of different anions as well. The complexity of these materials requires additional process steps to separate the desired compound from impurities and undesirable compounds. Furthermore, there is no indication or suggestion that the described method would be useful in other types of systems, particularly more complex systems, or in the recovery of other amphoteric compounds. The solubilities of different compounds containing amphoteric metals can vary significantly. For example, lead sulfate is only soluble in hot, concentrated sodium hydroxide solution, while zinc sulfate is very soluble in 25% NaOH, even at room temperature. The solubility of halides decreases significantly above about 35% caustic at room temperature.